A noun for “blood” appearing as an element in the name Agarwaen (S/210). It’s later etymology is unclear.
Conceptual Development: An earlier iteration of the name was N. {Iarvael >>} N. Iarwath “Blood-stained” from The Etymologies of the 1930s, where the element was N. iâr “blood” from the root ᴹ√YAR of the same meaning (Ety/YAR). Tolkien considered changing the root to ᴹ√YOR and the Noldorin form to iûr (EtyAC/YAR). This seems to be transient idea, since the name Iarwaeth “Bloodstained” appeared in the Grey Annals from the early 1950s (WJ/83) before ultimately being replaced by Agarwaen (WJ/142).
Neo-Sindarin: For purposes of Neo-Sindarin, I’d stick to the better-described sereg for “blood”.
A root in Quenya Notes (QN) from 1957 given as the basis for the “blood” words Q. serke and S. sereg as well as the flower name S. seregon “blood of stone” (PE17/184), a flower name that also appeared (untranslated) in The Silmarillion (S/203). It may replace the root ᴹ√YAR “blood” from The Etymologies of the 1930s with derivatives like ᴹQ. yár and N. iâr of the same meaning, the later an element in the surname of Túrin: N. Iarwath “Blood-stain” (Ety/YAR). In later Silmarillion drafts, it became Iarwaeth (WJ/83) and then S. Agarwaen “Blood-stained”, the form it took in the published version of The Silmarillion (S/210). The etymology of S. agar- “blood” is unclear.
Neo-Eldarin: For purposes of Neo-Eldarin, I’d stick to √SEREK = “blood”.